1. Field of the Invention
This invention relates to a new process for the production of polyisocyanates containing isocyanurate groups in the presence of a trimerization catalyst and carbon dioxide and to the polyisocyanates obtained by this process.
2. Background of the Invention
The products obtained by the process according to the invention are generally mixtures which, in addition to isocyanurate polyisocyanates having the idealized formula (II), contain oxadiazinetrione diisocyanates having the idealized formula (I) and iminooxadiazinedione triisocyanates corresponding to formula (III): ##STR1## In these formulas, x is the hydrocarbon radical of the (cyclo)aliphatic diisocyanate used for the production of the modified polyisocyanates. In addition to the triisocyanates and diisocyanates corresponding to the above general formulae, the end products of the process according to the invention may also contain higher homologs having more than one heterocyclic ring of the type mentioned per molecule.
The process for the production of such polyisocyanate mixtures is new. It is true that there are several known processes for the trimerization of organic isocyanates using strong organic bases in particular as catalysts (J. H.. Saunders and K. C. Frisch, Polyurethanes, Chemistry and Technology, pages 94 et seq, 1962). When organic diisocyanates are used and when the trimerization reaction is terminated before all of the isocyanate groups of the starting diisocyanate have reacted, isocyanurate polyisocyanates are obtained which are valuable starting materials for the production of polyurethanes, in particular as the polyisocyanate component in polyurethane coating compositions. The combination of this trimerization reaction with the production of the oxadiazinetrione polyisocyanates described in DE-AS 1 670 666, i.e., the production of polyisocyanate mixtures corresponding to the end products of the process according to the invention, has not previously been described. This is not surprising because, on the one hand, carbon dioxide, an acid anhydride, hinders the base-catalyzed trimerization reaction and, on the other hand, carbon dioxide has to be present in excess in the process according to DE-AS 1 670 666 to avoid the formation of troublesome secondary products which cause an unwanted increase in viscosity.
It has now surprisingly been found that it is possible in accordance with the process of the present invention, which is described in detail hereinafter, to advantageously combine both modification reactions in such a way as to obtain modified isocyanurate polyisocyanates whose composition may be varied. The polyisocyanates are distinguished by a number of advantageous properties such as colorlessness, low viscosity and high compatibility with polyhydroxyl compounds commonly used with isocyanurate polyisocyanates.